Explosives are defined as substances or materials that when initiated by friction, impact, shock, or heat undergo a chemical reaction that in turn expels a large amount of heat that exerting a high pressure on the surrounding environment [1]. Such explosives are part of a wider class of compounds, commonly referred to as energetic materials. Many of the explosives that have been synthesized contain a carbon core and oxidizing agents such as nitros, nitrate esters, and nitramines. Bonds such as nitrogen- nitrogen and nitrogen- oxygen bonds contain non- bonding or lone pairs electrons. These lone pair electrons create repulsion between the atoms. When these bonds break a vast amount of energy, nitrogen gas and carbon dioxide gas is released. In nitrogen gas and carbon dioxide gas, the non-bonding electrons are tied up in pi- bonds that are stable.
There are many ways to categorize explosives. One way is to designate explosives as high explosives or low explosives [1]. Typically, high explosives do not require confinement to explode and chemical reactions occur much more rapidly than in low explosives [1]. High explosives detonate at extremely high rates and include compounds such as TNT and RDX [1]. Typical detonation rates are between 1000 to 8500 m/s [1]. Low explosives undergo deflagration through surface burning [1]. Low explosives detonate at velocities much lower than high explosives. Detonation rates for low explosives range between a few cm/s up to about 400 m/s [1]. Low explosives include propellants, gunpowder, and smokeless powder [1].
Another way to categorize explosives is by how sensitive it is to mechanical or thermal stimuli [1]. Based on this category, explosives can be primary or secondary [1]. Primary explosives are extremely sensitive to impact, friction, or heat [1]. Typically, only a small amount of energy is needed to trigger initiation [1]. Primary explosives are often used in detonators and also as a means to trigger secondary explosives which are less sensitive [1]. Secondary explosives require a lot more energy to trigger initiation [1]. Due to less sensitivity than primary explosives, secondary explosives can be utilized in a wider variety of applications [1]. Additionally, secondary explosives are much easier to store and handle [1].
From a synthetic organic chemist’s viewpoint, the most efficient way to categorize explosives is by the functional groups that give these compounds explosive properties. As mentioned previously, most organic explosives contain nitrate ester, nitramine, or aliphatic or aromatic C-nitro functional groups. Due to the vast range of organic explosives, this paper will focus primarily on the synthesis and reactions of compounds containing aliphatic C-nitro groups and polynitropolycycloalkanes.
II. Aliphatic C- Nitro Compounds
The nitro group has been of interest to synthetic chemists for many years; it is probably the most widely studied functional group. This keen interest in nitro compounds stems from its explosive nature in many energetic materials. Aliphatic nitroalkanes can be placed into several categories. Three common categories are primary nitroalkanes, secondary nitroalkanes, and trimethyl nitroalkanes [2]. Primary nitroalkanes and secondary nitroalkanes contain acidic protons and typically are used in condensation related reactions to synthesize functionalized compounds [2]. Additionally, the nitro group is an excellent electron withdrawing group and the nitronate ion is resonance stabilized that adds to the acidity of primary nitroalkanes and secondary nitroalkanes [2] Primary alkanes and secondary alkanes are important building blocks for the synthesis of polynitropolycycloalkanes.
One method to synthesize nitroalkanes is through direct nitration of aliphatic hydrocarbons [3]. The synthesis of nitroalkanes can be done with nitric acid or nitrogen dioxide at high temperatures [3]. Titov (1963) proposed that the nitration occurs via a radical mechanism [3]. According to the experiments conducted by Titov, tertiary carbon centers underwent nitration most readily, followed by secondary and primary carbon centers [3]. Secondary and primary carbon centers were nitrated but with difficulty [3]. When nitric acid was introduced to alkanes without the presence of NO2, nitration did not occur, suggesting that the nitric acid was inert [3]. However, in the presence of NO2 at 300 C, nitration did occur [3]. Nitration with nitric acid in the presence of nitrogen oxides proceeded at a rate proportional to the concentration of NO2 present [3]. Frankel et al. (1957) studied the nitration of nitroolefins such as 2-nitro-1-pro- pene, 2-nitro-1-butenel and 2-nitro-2-butene [4]. These nitro compounds were reacted with 70% nitric acid under various conditions [4]. Although mixtures were obtained from the reactions, Frankel did obtain a solid product for the reaction 2-nitro-2-butene with nitric acid that was purified [4].The product was identified as 2,2,3-trinitrobutane as seen below in the reaction scheme [4].
Figure 1.1 2-nitro-2-butene was reacted with 70% nitric acid at 40 0C to yield 2,2,3-trinitrobutane. Upon addition of NaOH to the 2,2,3 trinitrobutane, 2,3 dinitrobutene was formed [3].
Emmons and Stevens (1957) discovered that alkenes react with dinitrogen pentoxide to form nitroalkene compounds and a mixture of other products such as β-nitro nitrates that are highly unstable [5]. Due to the instability of the β-nitro nitrates, corresponding alkenes form readily [5]. Additionally, alkenes can undergo reactions with silver nitrate and iodine to form nitroalkyl iodides [5]. This reaction seems to have the characteristics of nitryl iodide addition [6]. After the nitryl iodide addition, free radical attack occurs of the nitro on the double bond [6]. Finally, the radical is quenched with iodine [6]. The synthesis of nitroalkyl iodides is important because these compounds provide a convenient route for the formation of α-nitroalkenes.
Silver nitrate also plays a role in the synthesis of α-nitroesters [7]. Kornblum et al. (1955) studied the reaction of silver nitrate with α-haloesters that proved to be useful in the synthesis of α-nitroesters [7]. A mechanism was proposed that described a “pull-push” process involving the development of an electron deficiency at the carbon atom undergoing substitution [7]. The carbonium character of the transition state determined the outcome of the yield for this type of reaction [7]. The greater the yield of the carbonium character, the greater was the yield of the nitrite ester and the less was the yield of the nitro compound [7].
Alkenes can also react with nitric oxide and dinitro trioxide to produce dinitro and nitro-nitroso compounds [8]. Nitric oxide was reacted with tetrafluoroethylene at room temperature and atmospheric pressure for 48 hours in the dark and yielded tetrafluro-1-nitro-2-nitrosoethane (68%) [8]. When excess amounts of either nitric oxide or dinitrogen trioxide were used, the dinitroalkane was the main product of the reaction [8].
The direct oxidation of amines and the ozonation of alkyl and alkyl aryl amines are two another methods used to synthesize nitro compounds [9]. Potassium permanganate oxidizes tertiary amines into the desired nitro compounds [9]. Sollott and Gilbert (1980) reported that although the use of potassium permanganate was common in converting tertiary amines to respective nitro compounds, this oxidizing agent had not been used to make compounds with more than two nitro groups [9]. Sollot and Gilbert, were able to synthesize 1,3,5,7-tetra-nitroadamantane from its tetraamino derivative in a 45% yield [9]. As a result of this study, it became known that tetraamino derivatives successfully could be oxidized via potassium permanganate to form tetranitro compounds [9]. Mazur and Keinan (1977) successfully obtained nitro through the oxidation of primary amines by ozone [10]. Although using ozone to oxidize primaryaliphatic amines into nitro compounds proved to be effective, many side products formed that posed as a disadvantage [10]. As a result, side reactions were suppressed by dissolving the primary amine in silica gel [10]. Then, a stream of ozone in oxygen is passed through the solid at a temperature of -78 C [10]. More specifically, butylamine reacted with three equivalents of ozone to yield 2-nitrobutane [10]. The complete conversion of the amine into 2-nitrobutane suggested that the reaction between the amine and the ozone was instantaneous [10].
Furthermore, Wade and colleagues used ozone to synthesize 1,l bis(nitromethy1) cyclopropane attached from the corresponding diamine [11]. This product is a known energetic material. Specifically, the diamine contained water and was derived from a hydrochloric acid salt [11]. The wet diamine was dissolved in silica gel and excess ozone was passed through at -78 0C under anhydrous conditions [11]. Wade and colleagues reported that the yield of 1,l bis(nitromethy1) cyclopropane was between 20-28% depending on the grade of the silica [11].
Figure 1.3 The reaction scheme of the synthesis of 1,1 bis(nitromethyl) cyclopropane [11].
Nitro compounds also have been synthesized by the direct oxidation of isocyanate intermediates [12]. Typically, a series of reactions had to be carried out in order to obtain the desired nitro compound [12]. The reaction scheme involved an acid that was converted to an acylazide that then was converted to an isocyanate, that was followed by the formation of a carbamate that was converted into an amine and finally into a nitro compound [12]. Looking at this reaction scheme, it is safe to assume that the synthesis of nitro compounds this way was tedious and complicated. To simplify this route, a reagent would be used that could directly oxidize the isocyanate to the nitro compound [12]. Not only would this simplify the reaction, but would also avoid working with free amines. Eaton and Wicks (1988), used dimethyldioxirane in acetone as an oxidizing agent [12]. Primary, secondary, and tertiary aliphatic isocyanates were oxidized successfully into the corresponding nitro compounds in decent yields [12].
Sodium borohydride has been used as a reducing agent that converts β-nitro nitrates into nitroalkanes [13]. Previously, the conversion of 1-alkenes into terminal nitroalkanes had many limitations [13]. The reaction scheme consisted of taking an alkene converting it into a dinitroalkane or nitro alcohol and then using base to form the nitroalkene [13]. Then the nitroalkene was catalytically hydrogenated into a nitroalkane [13]. This reaction scheme posed several disadvantages. Among these disadvantages were the dimerization or polymerization of the nitroalkene and the possibility of further reducing the nitroalkene into an amine [13]. Larkin and Kreuz (1971) used sodium borohydride to reduce β-nitro nitrates into nitroalkanes providing a new route for synthesis without these disadvantages [13].
Additionally, oxidative nitration is a known effective means to synthesize gem-dinitroaliphatic compounds [14]. This process was discovered by Kaplan and Shechter in the early 1960s [14]. The salts of primary and secondary nitro compounds were converted into the corresponding gem- dinitro compounds by reacting the salts with silver nitrate and inorganic nitrites [14]. This was done in either neutral or basic media [14]. The mechanism that was proposed involved an addition complex that collapses and leads to oxidative addition of the nitrite anion to the nitronate and the reduction of silver [14]. The Kaplan- Schehter Reaction has many advantages. Advantages include relatively good yields of the desired products and avoiding the isolation of gem-nitronitronate salts that can explode upon impact or friction [14]. However, one major disadvantage is the cost of the silver nitrate which makes it difficult to work with on larger scales for industrial purposes.
Figure 1.2 Proposed mechanism for the Kaplan- Schehter Reaction or oxidative nitration [14]. reactants Addition reactions are useful in synthesizing nitro compounds [15]. One important addition reaction in synthetic organic chemistry is the Michael reaction [15]. Typically, in this reaction, a Michael donor undergoes conjugate addition to the Michael acceptor that is usually, but not limited to an α, β unsaturated ketone or aldehyde [15]. Nitroalkanes are known good Michael donors and nitroalkenes are good Michael acceptors [15]. Thus, nitronate ions can be added across the double bond of an electron deficient alkene. Generally, the product of a Michael Reaction depends on the number of acidic hydrogens that are present in the nitroalkane substrate as observed by Gilligan and Graff [16]. Nitroform, a strong acid, was reacted with unsaturated ketones to give trinitromethyl compounds [16]. More specifically, nitroform in 50% aqueous methanol solution was added dropwise to a solution of 2-cyclohexanone ethylene ketal and to a solution of 2-cyclopentanone ethylene ketal to form 3-trinitromethylhexanone and 3-trinitromethylpentanone [16]. These compounds proved to be efficient energetic materials.
Conjugate 1,4 additions also play an important role in the synthesis of polynitroaliphatic compounds. Frankel and colleagues reacted nitroform with nitroethene to form 1,1,1,3-tetranitropropane [17]. Feuer and co-workers investigated Michael reactions involving α,α,ω,ω tetranitroalkenes and their bis-methylol derivatives [18]. These chemists were interested in α,α,ω,ω tetranitroalkenes due to potential use for the synthesis of energetic oligomers [18]. Most α,α,ω,ω tetranitroalkenes react with two equivalents of Michael acceptor to yield the bis-methylol derivatives [18]. These reactions were done in the presence of base that formed the nitronate anion and formaldehyde [18]. This was done as a safety precaution to prevent the formation of polynitroaliphatic nitronate salts that can explode upon friction or impact [18]. One disadvantage with Michael additions is that polymerization can occur [15].
III. Polynitropolycycloalkanes
Polynitropolycycloalkanes are energetic compounds that are made of strained ring structures or caged acyclic carbon skeletons that contain nitro groups. The synthesis of these compounds is more complex than that of simple aliphatic compounds. Despite the complexity of the synthetic routes that lead to polynitropolycycloalkanes, the same chemistry is employed to introduce C-nitro functionality. The combustion of the carbon skeleton of aliphatic C- nitro compounds is the source of their energy. Scientists want to synthesize energetic materials that obtain their energy not only through their carbon skeleton but also relief of angle strain. The relief of angle strain releases a great amount of energy. The structures of caged or strained compounds would result in higher crystal density due to restriction of motion that would in turn tremendously improve explosive performance. However, with greater explosive performance, chemical and thermal stability can be compromised.
Energetic materials with strained rings in their structures include polynitro derivatives of cyclopropane and spirocyclopropane [11]. These polynitro derivatives are of relatively low molecular weight and were synthesized by Wade and colleagues [11]. Several dinitrospiropentanes were prepared from key intermediate 1,1-bis(nitromethyl)cyclopropane [11]. This intermediate underwent oxidative cyclization to form 1,2 dinitrospiropentane in a 43% yield [11]. Furthermore, 1,2 dinitrospiropentane was functionalized with other groups to yield other dinitrispiropentanes. Interestingly enough, only the trans isomers were observed [11].
Archibald and co-workers investigated the synthesis of polynitrocyclobutanes and their derivatives [19]. Aminocyclobutanes were oxidized with m-chloroperbenzoic acid in refluxing dichloromethane to form the nitro derivatives [19]. More specifically, 1-aminocyclobutane was oxidized into 1-nitrocyclobutane in a 34% yield [19]. Furthermore, 1,1,3,3- tetranitrocyclobutane and 1,3,3- trinitrocyclobutane were synthesized [19]. This was done by oxidative nitration of the 1,3 dinitrocyclobutane salt with a silver nitrate and sodium nitrite mixture [19]. Tetranitrocyclobutane was stable thermally until its reported melting point of 165 C [19]. Contrary to tetranitrocyclobutane, trinitrocyclobutane was unstable and was not isolated [19]. Archibald and colleagues also synthesized 5,10- Dinitrodispiro[3.1.3.1]decane from its corresponding oxime and then underwent oxidative nitration to form 5,5,10,10- Tetranitrodispiro[3.1.3.1]decane in a relatively high yield (64%) [19].
Other caged compounds of interest are organic compounds known as cubanes. This interest stems from the structure of cubane that requires the placement of eight methane carbon atoms at the vertices of a cube [20]. One would safely assume that placing these carbon atoms in this specific arrangement would cause the molecule to be highly strained [20]. As observed by Griffin, the total strain energy is 166 kcal/mol for cubane [20]. Although there is considerable strain in this compound, it is remarkably stable [20]. Cubanes are by nature highly energetic compounds due to high molecular strain; to functionalize them with nitro groups would increase the energetics of the compound tremendously.
During the early 1980s, Eaton and colleagues functionalized cubane with nitro groups [21]. Their goal was to synthesize energetic compounds that had good thermal stability and high density [21]. Before Eaton and coworkers, the synthesis of cubanes with nitro functional groups was not reported in the literature [21]. These chemists undertook the synthesis of 1,4-dinitrocubane [21]. The literature has reported that cubane begins to decompose slowly at 200 C [20]. However, it has been suggested that the nitro groups which are excellent withdrawing groups stabilize the compound [21]. Increased stabilization of the cubane system would result in increased thermal stability making these compounds of high interest for military and civilian applications. The dinitrocubane was synthesized via two ways. The first method involved cubane- 1,4-dicarboxylic acid that was treated with di- phenylphosphoryl azide and triethylamine in tert-butyl alcohol to give 1,4-bis[(tert-butoxycarbony1)- aminolcubane [21]. After hydrolysis and decarboxylation and finally oxidation of the resulting amine was converted into the desired compound, 1,4 dinitrocubane [21]. This method was extremely useful because it avoided dealing with diacyl azide, a very sensitive compound [21]. The second method involved oxidation of the diamine and its hydrochloride salt using dimethyldioxirane to form 1,4 dinitrocubane in a very good yield [21].
Eaton and colleagues were also able to make 1,3,5 trinitrocubane and 1,3,5,7 tetranitrocubane [21].These two compounds were synthesized through stepwise substitution of the cubane core [21]. Amide functionality allowed for othro- metalation of adjacent positions, but the synthesis of the precursors was not ideal because of the formation of organometallic intermediates that are extremely toxic [21]. As a result, Eaton and colleagues tried a different method that allowed for the synthesis of 1,3,5,7 tetranitrocubane using cubane- 1,3,5,7-tetracarboxylic acid chloride as a precursor [21]. Cubane- 1,3,5,7-tetracarboxylic acid chloride was treated with dimethylsilylazide yielding tetraacylazide, a notoriously explosive compound [21]. When this compound is heated in a refluxing solution of chloroform the tetraisocyanate derivative is formed [21]. The tetraisocyanate derivative was isolated and oxidized with dimethyldioxirane in wet acetone that gave the desired compound, 1,3,5,7-tetranitrocubane [21]. Eaton and colleagues reported a density of 1.814 g/cm3 for this compound and a melting point of 270 C [20], making 1,3,5,7- tetranitrocubane an effective explosive. Additionally, 1,2,3,4,5,7-hexanitrocubane was synthesized [21]. This compound has been reported to be the most highly nitrated cubane made in recent years [21].
IV. Conclusion
Aliphatic C- nitro compounds and polynitripolycycloalkanes are of tremendous importance in the world of energetic materials. The effectiveness of an explosive depends on several factors. Three very important factors are the energetics of the decomposition reaction, the number of moles of molecular wright of the gaseous products, and the density of the compound [22]. Typically, the more moles of an explosive that can be packed into a shell that has limited volume, the more efficient is the explosive [22]. Furthermore, the rate of energy release is proportional to the square of the density [22]. As a result, an effective explosive will have a high density. Although these properties characterize explosives as effective, it is important to also take safety, stability, and reliability into consideration especially since explosives have a plethora of applications in the military world. Explosives have existed for hundreds of years and the literature describes the chemistry behind explosives in great detail, but chemists continue to study the chemistry of explosives in an effort to achieve maximum performance. More specifically, chemists and engineers seek to synthesize heat resistant explosives that can be implemented in warheads of high-speed missiles, insensitive high explosives to prevent accidental detonation or initiation, and melt-castable explosives [22]. Melt-castable explosives, such as TNT are extremely useful for military applications because these energetic materials can be melted and molded into desired shapes and dimensions [22]. Thus, high quality explosives can be synthesized using well-controlled casting parameters [22].
Although energetic compounds have played an imperative role in the military world, it is worthwhile to mention the impact that energetic compounds have had on civilian life. Although extremely dangerous when not handled by the right individuals, energetic compounds have increased the standard of living in the past few hundred years. The houses we live in, the highways we drive on, the bridges we cross to get to our destinations have all been built from materials that have been obtained by using explosives. Important minerals such as copper and zinc have been extracted from the ground through the use of explosives. Even the silica used in flash chromatography has been extracted with the use of explosives! In conclusion, energetic materials should be handled by highly trained individuals and must be handled with extreme care.
References
(1). Department of the Army and Airforce. Military Explosives. Washington, D.C., 1967. Link
(3). Titov, A.I. (1963). "The Free Radical Mechanism of Nitration." Tetrahedron. 19,557-580.Link
(4). Frankel, M.B., Klager, K. (1958). "Nitration of Nitroolefins with Nitric Acid.” Journal of Organic Chemistry. 23, 494-495.Link
(5). Emmons, W.D., Stevens, T.E. (1957). "The Dinitrogen Pentoxide- Olefin Reaction." Journal of the American Chemical Society . 79, 6008-6014. Link
(6). Hassner, A., et al. (1969). "Addition of Nitryl Iodide to Olefins." Journal of Organic Chemistry. 34, (9), 2628-2632.Link
(7). Kornblum, N., et al. (1955). "The Reaction of Silver Nitrite with a-Haloesters." Journal of the American Chemical Society.77, (24), 6654-6655. Link
(8). Birchall, J.M., et al. (1962). "Perfluoroalkyl derivatives of nitrogen. Part X. The reaction of nitric oxide with tetrafluoroethylene and formation of a nitrosopolymer." Journal of the Chemical Society. 3021-3022.Link
(9). Sollott, G. P., Gilbert, E. E. (1980). “A facile route to 1,3,5,7-tetraaminoadamantane. Synthesis of 1,3,5,7-tetranitroadamantane.” The Journal of Organic Chemistry. 45, (26), 5405–5408.Link
(10). Keinan, E., Mazur, Y. (1977). “Dry ozonation of amines. Conversion of primary amines to nitro compounds.” The Journal of Organic Chemistry. 42, (5), 844–847. Link
(11). Wade, P. A., Kondracki, P. A., Carroll, P. J. (1991). “Polynitro-substituted strained-ring compounds. 2. 1,2-Dinitrospiropentanes.”Journal of the American Chemical Society. 113, (23), 8807–8811.Link
(12). Eaton, P. E., Wicks, G. E. (1988). “Conversion of isocyanates to nitro compounds with dimethyldioxirane in wet acetone.” The Journal of Organic Chemistry. 53, (22), 5353–5355.Link
(13). Larkin, J. M., Kreuz, K. L. (1971). “Conversion of vicinal nitro nitrates to nitroalkanes with sodium borohydride.” The Journal of Organic Chemistry. 36, (17), 2574–2575.Link
(14). Kaplan, R. B., Shechter, H. (1961). “ A new general reaction for preparing gem dinitro compounds: Oxidative Nitration.” Journal of the American Chemical Society. 83, (16), 3535–3536.Link
(15). Smith, M.B., March, J. March's Advanced Organic Chemistry. Reactions, Mechanisms, and Structure, 6th ed; John Wiley & Sons Inc., Hoboken, 2007.
(16). Graff, M., Gilligan, W. H. (1968). “Reaction of unsaturated ketones with polynitro addends.” The Journal of Organic Chemistry. 33, (3), 1247–1249.Link
(17). Frankel, M.B., (1963). "Synthesis and Reactions of Trinitromethyl Compounds." Tetrahedron. 19, Supp 1, 213-217.Link
(18). Feuer, H., Leston, G., Miller, R., Nielsen, A. T. (1963). “Michael-type Reactions with α,α,ι,ι-Tetranitroalkanes.” The Journal of Organic Chemistry. 28, (2), 339–344.Link
(19). Archibald, T. G., Garver, L. C., Baum, K., Cohen, M. C. (1989). “Synthesis of polynitrocyclobutane derivatives.” The Journal of Organic Chemistry. 54, (12), 2869–2873.Link
(20). Griffin, G. W., Marchand, A. P. (1989). “Synthesis and chemistry of cubanes.” Chemical Reviews. 89, (5), 997–1010.Link
(21). Eaton, P.E., et al. (1997). "Synthesis and Chemistry of 1,3,5,7-Tetranitrocubane Including Measurement of Its Acidity, Formation of o-Nitro Anions, and the First Preparations of Pentanitrocubane and Hexanitrocubane." Journal of the American Chemical Society.119, (41), 9591-9602.Link
(22). Agrawal, J.P. (1998). "Recent Trends in High-Energy Materials." Progress in Energy and Combustion Science.24, (1), 1-30.Link
Final Paper
Aliphatic C- Nitros & Polynitropolycycloalkanes
I. Introduction
Explosives are defined as substances or materials that when initiated by friction, impact, shock, or heat undergo a chemical reaction that in turn expels a large amount of heat that exerting a high pressure on the surrounding environment [1]. Such explosives are part of a wider class of compounds, commonly referred to as energetic materials. Many of the explosives that have been synthesized contain a carbon core and oxidizing agents such as nitros, nitrate esters, and nitramines. Bonds such as nitrogen- nitrogen and nitrogen- oxygen bonds contain non- bonding or lone pairs electrons. These lone pair electrons create repulsion between the atoms. When these bonds break a vast amount of energy, nitrogen gas and carbon dioxide gas is released. In nitrogen gas and carbon dioxide gas, the non-bonding electrons are tied up in pi- bonds that are stable.
There are many ways to categorize explosives. One way is to designate explosives as high explosives or low explosives [1]. Typically, high explosives do not require confinement to explode and chemical reactions occur much more rapidly than in low explosives [1]. High explosives detonate at extremely high rates and include compounds such as TNT and RDX [1]. Typical detonation rates are between 1000 to 8500 m/s [1]. Low explosives undergo deflagration through surface burning [1]. Low explosives detonate at velocities much lower than high explosives. Detonation rates for low explosives range between a few cm/s up to about 400 m/s [1]. Low explosives include propellants, gunpowder, and smokeless powder [1].
Another way to categorize explosives is by how sensitive it is to mechanical or thermal stimuli [1]. Based on this category, explosives can be primary or secondary [1]. Primary explosives are extremely sensitive to impact, friction, or heat [1]. Typically, only a small amount of energy is needed to trigger initiation [1]. Primary explosives are often used in detonators and also as a means to trigger secondary explosives which are less sensitive [1]. Secondary explosives require a lot more energy to trigger initiation [1]. Due to less sensitivity than primary explosives, secondary explosives can be utilized in a wider variety of applications [1]. Additionally, secondary explosives are much easier to store and handle [1].
From a synthetic organic chemist’s viewpoint, the most efficient way to categorize explosives is by the functional groups that give these compounds explosive properties. As mentioned previously, most organic explosives contain nitrate ester, nitramine, or aliphatic or aromatic C-nitro functional groups. Due to the vast range of organic explosives, this paper will focus primarily on the synthesis and reactions of compounds containing aliphatic C-nitro groups and polynitropolycycloalkanes.
II. Aliphatic C- Nitro Compounds
The nitro group has been of interest to synthetic chemists for many years; it is probably the most widely studied functional group. This keen interest in nitro compounds stems from its explosive nature in many energetic materials. Aliphatic nitroalkanes can be placed into several categories. Three common categories are primary nitroalkanes, secondary nitroalkanes, and trimethyl nitroalkanes [2]. Primary nitroalkanes and secondary nitroalkanes contain acidic protons and typically are used in condensation related reactions to synthesize functionalized compounds [2]. Additionally, the nitro group is an excellent electron withdrawing group and the nitronate ion is resonance stabilized that adds to the acidity of primary nitroalkanes and secondary nitroalkanes [2] Primary alkanes and secondary alkanes are important building blocks for the synthesis of polynitropolycycloalkanes.
One method to synthesize nitroalkanes is through direct nitration of aliphatic hydrocarbons [3]. The synthesis of nitroalkanes can be done with nitric acid or nitrogen dioxide at high temperatures [3]. Titov (1963) proposed that the nitration occurs via a radical mechanism [3]. According to the experiments conducted by Titov, tertiary carbon centers underwent nitration most readily, followed by secondary and primary carbon centers [3]. Secondary and primary carbon centers were nitrated but with difficulty [3]. When nitric acid was introduced to alkanes without the presence of NO2, nitration did not occur, suggesting that the nitric acid was inert [3]. However, in the presence of NO2 at 300 C, nitration did occur [3]. Nitration with nitric acid in the presence of nitrogen oxides proceeded at a rate proportional to the concentration of NO2 present [3]. Frankel et al. (1957) studied the nitration of nitroolefins such as 2-nitro-1-pro- pene, 2-nitro-1-butenel and 2-nitro-2-butene [4]. These nitro compounds were reacted with 70% nitric acid under various conditions [4]. Although mixtures were obtained from the reactions, Frankel did obtain a solid product for the reaction 2-nitro-2-butene with nitric acid that was purified [4].The product was identified as 2,2,3-trinitrobutane as seen below in the reaction scheme [4].
Figure 1.1 2-nitro-2-butene was reacted with 70% nitric acid at 40 0C to yield 2,2,3-trinitrobutane. Upon addition of NaOH to the 2,2,3 trinitrobutane, 2,3 dinitrobutene was formed [3].
Emmons and Stevens (1957) discovered that alkenes react with dinitrogen pentoxide to form nitroalkene compounds and a mixture of other products such as β-nitro nitrates that are highly unstable [5]. Due to the instability of the β-nitro nitrates, corresponding alkenes form readily [5]. Additionally, alkenes can undergo reactions with silver nitrate and iodine to form nitroalkyl iodides [5]. This reaction seems to have the characteristics of nitryl iodide addition [6]. After the nitryl iodide addition, free radical attack occurs of the nitro on the double bond [6]. Finally, the radical is quenched with iodine [6]. The synthesis of nitroalkyl iodides is important because these compounds provide a convenient route for the formation of α-nitroalkenes.
Silver nitrate also plays a role in the synthesis of α-nitroesters [7]. Kornblum et al. (1955) studied the reaction of silver nitrate with α-haloesters that proved to be useful in the synthesis of α-nitroesters [7]. A mechanism was proposed that described a “pull-push” process involving the development of an electron deficiency at the carbon atom undergoing substitution [7]. The carbonium character of the transition state determined the outcome of the yield for this type of reaction [7]. The greater the yield of the carbonium character, the greater was the yield of the nitrite ester and the less was the yield of the nitro compound [7].
Alkenes can also react with nitric oxide and dinitro trioxide to produce dinitro and nitro-nitroso compounds [8]. Nitric oxide was reacted with tetrafluoroethylene at room temperature and atmospheric pressure for 48 hours in the dark and yielded tetrafluro-1-nitro-2-nitrosoethane (68%) [8]. When excess amounts of either nitric oxide or dinitrogen trioxide were used, the dinitroalkane was the main product of the reaction [8].
The direct oxidation of amines and the ozonation of alkyl and alkyl aryl amines are two another methods used to synthesize nitro compounds [9]. Potassium permanganate oxidizes tertiary amines into the desired nitro compounds [9]. Sollott and Gilbert (1980) reported that although the use of potassium permanganate was common in converting tertiary amines to respective nitro compounds, this oxidizing agent had not been used to make compounds with more than two nitro groups [9]. Sollot and Gilbert, were able to synthesize 1,3,5,7-tetra-nitroadamantane from its tetraamino derivative in a 45% yield [9]. As a result of this study, it became known that tetraamino derivatives successfully could be oxidized via potassium permanganate to form tetranitro compounds [9]. Mazur and Keinan (1977) successfully obtained nitro through the oxidation of primary amines by ozone [10]. Although using ozone to oxidize primaryaliphatic amines into nitro compounds proved to be effective, many side products formed that posed as a disadvantage [10]. As a result, side reactions were suppressed by dissolving the primary amine in silica gel [10]. Then, a stream of ozone in oxygen is passed through the solid at a temperature of -78 C [10]. More specifically, butylamine reacted with three equivalents of ozone to yield 2-nitrobutane [10]. The complete conversion of the amine into 2-nitrobutane suggested that the reaction between the amine and the ozone was instantaneous [10].
Furthermore, Wade and colleagues used ozone to synthesize 1,l bis(nitromethy1) cyclopropane attached from the corresponding diamine [11]. This product is a known energetic material. Specifically, the diamine contained water and was derived from a hydrochloric acid salt [11]. The wet diamine was dissolved in silica gel and excess ozone was passed through at -78 0C under anhydrous conditions [11]. Wade and colleagues reported that the yield of 1,l bis(nitromethy1) cyclopropane was between 20-28% depending on the grade of the silica [11].
Figure 1.3 The reaction scheme of the synthesis of 1,1 bis(nitromethyl) cyclopropane [11].
Nitro compounds also have been synthesized by the direct oxidation of isocyanate intermediates [12]. Typically, a series of reactions had to be carried out in order to obtain the desired nitro compound [12]. The reaction scheme involved an acid that was converted to an acylazide that then was converted to an isocyanate, that was followed by the formation of a carbamate that was converted into an amine and finally into a nitro compound [12]. Looking at this reaction scheme, it is safe to assume that the synthesis of nitro compounds this way was tedious and complicated. To simplify this route, a reagent would be used that could directly oxidize the isocyanate to the nitro compound [12]. Not only would this simplify the reaction, but would also avoid working with free amines. Eaton and Wicks (1988), used dimethyldioxirane in acetone as an oxidizing agent [12]. Primary, secondary, and tertiary aliphatic isocyanates were oxidized successfully into the corresponding nitro compounds in decent yields [12].
Sodium borohydride has been used as a reducing agent that converts β-nitro nitrates into nitroalkanes [13]. Previously, the conversion of 1-alkenes into terminal nitroalkanes had many limitations [13]. The reaction scheme consisted of taking an alkene converting it into a dinitroalkane or nitro alcohol and then using base to form the nitroalkene [13]. Then the nitroalkene was catalytically hydrogenated into a nitroalkane [13]. This reaction scheme posed several disadvantages. Among these disadvantages were the dimerization or polymerization of the nitroalkene and the possibility of further reducing the nitroalkene into an amine [13]. Larkin and Kreuz (1971) used sodium borohydride to reduce β-nitro nitrates into nitroalkanes providing a new route for synthesis without these disadvantages [13].
Additionally, oxidative nitration is a known effective means to synthesize gem-dinitroaliphatic compounds [14]. This process was discovered by Kaplan and Shechter in the early 1960s [14]. The salts of primary and secondary nitro compounds were converted into the corresponding gem- dinitro compounds by reacting the salts with silver nitrate and inorganic nitrites [14]. This was done in either neutral or basic media [14]. The mechanism that was proposed involved an addition complex that collapses and leads to oxidative addition of the nitrite anion to the nitronate and the reduction of silver [14]. The Kaplan- Schehter Reaction has many advantages. Advantages include relatively good yields of the desired products and avoiding the isolation of gem-nitronitronate salts that can explode upon impact or friction [14]. However, one major disadvantage is the cost of the silver nitrate which makes it difficult to work with on larger scales for industrial purposes.
Figure 1.2 Proposed mechanism for the Kaplan- Schehter Reaction or oxidative nitration [14].
reactants
Addition reactions are useful in synthesizing nitro compounds [15]. One important addition reaction in synthetic organic chemistry is the Michael reaction [15]. Typically, in this reaction, a Michael donor undergoes conjugate addition to the Michael acceptor that is usually, but not limited to an α, β unsaturated ketone or aldehyde [15]. Nitroalkanes are known good Michael donors and nitroalkenes are good Michael acceptors [15]. Thus, nitronate ions can be added across the double bond of an electron deficient alkene. Generally, the product of a Michael Reaction depends on the number of acidic hydrogens that are present in the nitroalkane substrate as observed by Gilligan and Graff [16]. Nitroform, a strong acid, was reacted with unsaturated ketones to give trinitromethyl compounds [16]. More specifically, nitroform in 50% aqueous methanol solution was added dropwise to a solution of 2-cyclohexanone ethylene ketal and to a solution of 2-cyclopentanone ethylene ketal to form 3-trinitromethylhexanone and 3-trinitromethylpentanone [16]. These compounds proved to be efficient energetic materials.
Conjugate 1,4 additions also play an important role in the synthesis of polynitroaliphatic compounds. Frankel and colleagues reacted nitroform with nitroethene to form 1,1,1,3-tetranitropropane [17]. Feuer and co-workers investigated Michael reactions involving α,α,ω,ω tetranitroalkenes and their bis-methylol derivatives [18]. These chemists were interested in α,α,ω,ω tetranitroalkenes due to potential use for the synthesis of energetic oligomers [18]. Most α,α,ω,ω tetranitroalkenes react with two equivalents of Michael acceptor to yield the bis-methylol derivatives [18]. These reactions were done in the presence of base that formed the nitronate anion and formaldehyde [18]. This was done as a safety precaution to prevent the formation of polynitroaliphatic nitronate salts that can explode upon friction or impact [18]. One disadvantage with Michael additions is that polymerization can occur [15].
III. Polynitropolycycloalkanes
Polynitropolycycloalkanes are energetic compounds that are made of strained ring structures or caged acyclic carbon skeletons that contain nitro groups. The synthesis of these compounds is more complex than that of simple aliphatic compounds. Despite the complexity of the synthetic routes that lead to polynitropolycycloalkanes, the same chemistry is employed to introduce C-nitro functionality. The combustion of the carbon skeleton of aliphatic C- nitro compounds is the source of their energy. Scientists want to synthesize energetic materials that obtain their energy not only through their carbon skeleton but also relief of angle strain. The relief of angle strain releases a great amount of energy. The structures of caged or strained compounds would result in higher crystal density due to restriction of motion that would in turn tremendously improve explosive performance. However, with greater explosive performance, chemical and thermal stability can be compromised.
Energetic materials with strained rings in their structures include polynitro derivatives of cyclopropane and spirocyclopropane [11]. These polynitro derivatives are of relatively low molecular weight and were synthesized by Wade and colleagues [11]. Several dinitrospiropentanes were prepared from key intermediate 1,1-bis(nitromethyl)cyclopropane [11]. This intermediate underwent oxidative cyclization to form 1,2 dinitrospiropentane in a 43% yield [11]. Furthermore, 1,2 dinitrospiropentane was functionalized with other groups to yield other dinitrispiropentanes. Interestingly enough, only the trans isomers were observed [11].
Archibald and co-workers investigated the synthesis of polynitrocyclobutanes and their derivatives [19]. Aminocyclobutanes were oxidized with m-chloroperbenzoic acid in refluxing dichloromethane to form the nitro derivatives [19]. More specifically, 1-aminocyclobutane was oxidized into 1-nitrocyclobutane in a 34% yield [19]. Furthermore, 1,1,3,3- tetranitrocyclobutane and 1,3,3- trinitrocyclobutane were synthesized [19]. This was done by oxidative nitration of the 1,3 dinitrocyclobutane salt with a silver nitrate and sodium nitrite mixture [19]. Tetranitrocyclobutane was stable thermally until its reported melting point of 165 C [19]. Contrary to tetranitrocyclobutane, trinitrocyclobutane was unstable and was not isolated [19]. Archibald and colleagues also synthesized 5,10- Dinitrodispiro[3.1.3.1]decane from its corresponding oxime and then underwent oxidative nitration to form 5,5,10,10- Tetranitrodispiro[3.1.3.1]decane in a relatively high yield (64%) [19].
Other caged compounds of interest are organic compounds known as cubanes. This interest stems from the structure of cubane that requires the placement of eight methane carbon atoms at the vertices of a cube [20]. One would safely assume that placing these carbon atoms in this specific arrangement would cause the molecule to be highly strained [20]. As observed by Griffin, the total strain energy is 166 kcal/mol for cubane [20]. Although there is considerable strain in this compound, it is remarkably stable [20]. Cubanes are by nature highly energetic compounds due to high molecular strain; to functionalize them with nitro groups would increase the energetics of the compound tremendously.
During the early 1980s, Eaton and colleagues functionalized cubane with nitro groups [21]. Their goal was to synthesize energetic compounds that had good thermal stability and high density [21]. Before Eaton and coworkers, the synthesis of cubanes with nitro functional groups was not reported in the literature [21]. These chemists undertook the synthesis of 1,4-dinitrocubane [21]. The literature has reported that cubane begins to decompose slowly at 200 C [20]. However, it has been suggested that the nitro groups which are excellent withdrawing groups stabilize the compound [21]. Increased stabilization of the cubane system would result in increased thermal stability making these compounds of high interest for military and civilian applications. The dinitrocubane was synthesized via two ways. The first method involved cubane- 1,4-dicarboxylic acid that was treated with di- phenylphosphoryl azide and triethylamine in tert-butyl alcohol to give 1,4-bis[(tert-butoxycarbony1)- aminolcubane [21]. After hydrolysis and decarboxylation and finally oxidation of the resulting amine was converted into the desired compound, 1,4 dinitrocubane [21]. This method was extremely useful because it avoided dealing with diacyl azide, a very sensitive compound [21]. The second method involved oxidation of the diamine and its hydrochloride salt using dimethyldioxirane to form 1,4 dinitrocubane in a very good yield [21].
Eaton and colleagues were also able to make 1,3,5 trinitrocubane and 1,3,5,7 tetranitrocubane [21].These two compounds were synthesized through stepwise substitution of the cubane core [21]. Amide functionality allowed for othro- metalation of adjacent positions, but the synthesis of the precursors was not ideal because of the formation of organometallic intermediates that are extremely toxic [21]. As a result, Eaton and colleagues tried a different method that allowed for the synthesis of 1,3,5,7 tetranitrocubane using cubane- 1,3,5,7-tetracarboxylic acid chloride as a precursor [21]. Cubane- 1,3,5,7-tetracarboxylic acid chloride was treated with dimethylsilylazide yielding tetraacylazide, a notoriously explosive compound [21]. When this compound is heated in a refluxing solution of chloroform the tetraisocyanate derivative is formed [21]. The tetraisocyanate derivative was isolated and oxidized with dimethyldioxirane in wet acetone that gave the desired compound, 1,3,5,7-tetranitrocubane [21]. Eaton and colleagues reported a density of 1.814 g/cm3 for this compound and a melting point of 270 C [20], making 1,3,5,7- tetranitrocubane an effective explosive. Additionally, 1,2,3,4,5,7-hexanitrocubane was synthesized [21]. This compound has been reported to be the most highly nitrated cubane made in recent years [21].
IV. Conclusion
Aliphatic C- nitro compounds and polynitripolycycloalkanes are of tremendous importance in the world of energetic materials. The effectiveness of an explosive depends on several factors. Three very important factors are the energetics of the decomposition reaction, the number of moles of molecular wright of the gaseous products, and the density of the compound [22]. Typically, the more moles of an explosive that can be packed into a shell that has limited volume, the more efficient is the explosive [22]. Furthermore, the rate of energy release is proportional to the square of the density [22]. As a result, an effective explosive will have a high density. Although these properties characterize explosives as effective, it is important to also take safety, stability, and reliability into consideration especially since explosives have a plethora of applications in the military world. Explosives have existed for hundreds of years and the literature describes the chemistry behind explosives in great detail, but chemists continue to study the chemistry of explosives in an effort to achieve maximum performance. More specifically, chemists and engineers seek to synthesize heat resistant explosives that can be implemented in warheads of high-speed missiles, insensitive high explosives to prevent accidental detonation or initiation, and melt-castable explosives [22]. Melt-castable explosives, such as TNT are extremely useful for military applications because these energetic materials can be melted and molded into desired shapes and dimensions [22]. Thus, high quality explosives can be synthesized using well-controlled casting parameters [22].
Although energetic compounds have played an imperative role in the military world, it is worthwhile to mention the impact that energetic compounds have had on civilian life. Although extremely dangerous when not handled by the right individuals, energetic compounds have increased the standard of living in the past few hundred years. The houses we live in, the highways we drive on, the bridges we cross to get to our destinations have all been built from materials that have been obtained by using explosives. Important minerals such as copper and zinc have been extracted from the ground through the use of explosives. Even the silica used in flash chromatography has been extracted with the use of explosives! In conclusion, energetic materials should be handled by highly trained individuals and must be handled with extreme care.
References
(1). Department of the Army and Airforce. Military Explosives. Washington, D.C., 1967. Link
(2). Markofsky, S.B., Grace, W.G. &Co.Nitro Compounds, Aliphatic. Columbia, 2005. Link
(3). Titov, A.I. (1963). "The Free Radical Mechanism of Nitration." Tetrahedron. 19,557-580.Link
(4). Frankel, M.B., Klager, K. (1958). "Nitration of Nitroolefins with Nitric Acid.” Journal of Organic Chemistry. 23, 494-495.Link
(5). Emmons, W.D., Stevens, T.E. (1957). "The Dinitrogen Pentoxide- Olefin Reaction." Journal of the American Chemical Society . 79, 6008-6014. Link
(6). Hassner, A., et al. (1969). "Addition of Nitryl Iodide to Olefins." Journal of Organic Chemistry. 34, (9), 2628-2632.Link
(7). Kornblum, N., et al. (1955). "The Reaction of Silver Nitrite with a-Haloesters." Journal of the American Chemical Society. 77, (24), 6654-6655. Link
(8). Birchall, J.M., et al. (1962). "Perfluoroalkyl derivatives of nitrogen. Part X. The reaction of nitric oxide with tetrafluoroethylene and formation of a nitrosopolymer." Journal of the Chemical Society. 3021-3022.Link
(9). Sollott, G. P., Gilbert, E. E. (1980). “A facile route to 1,3,5,7-tetraaminoadamantane. Synthesis of 1,3,5,7-tetranitroadamantane.” The Journal of Organic Chemistry. 45, (26), 5405–5408.Link
(10). Keinan, E., Mazur, Y. (1977). “Dry ozonation of amines. Conversion of primary amines to nitro compounds.” The Journal of Organic Chemistry. 42, (5), 844–847. Link
(11). Wade, P. A., Kondracki, P. A., Carroll, P. J. (1991). “Polynitro-substituted strained-ring compounds. 2. 1,2-Dinitrospiropentanes.”Journal of the American Chemical Society. 113, (23), 8807–8811.Link
(12). Eaton, P. E., Wicks, G. E. (1988). “Conversion of isocyanates to nitro compounds with dimethyldioxirane in wet acetone.” The Journal of Organic Chemistry. 53, (22), 5353–5355.Link
(13). Larkin, J. M., Kreuz, K. L. (1971). “Conversion of vicinal nitro nitrates to nitroalkanes with sodium borohydride.” The Journal of Organic Chemistry. 36, (17), 2574–2575.Link
(14). Kaplan, R. B., Shechter, H. (1961). “ A new general reaction for preparing gem dinitro compounds: Oxidative Nitration.” Journal of the American Chemical Society. 83, (16), 3535–3536.Link
(15). Smith, M.B., March, J. March's Advanced Organic Chemistry. Reactions, Mechanisms, and Structure, 6th ed; John Wiley & Sons Inc., Hoboken, 2007.
(16). Graff, M., Gilligan, W. H. (1968). “Reaction of unsaturated ketones with polynitro addends.” The Journal of Organic Chemistry. 33, (3), 1247–1249.Link
(17). Frankel, M.B., (1963). "Synthesis and Reactions of Trinitromethyl Compounds." Tetrahedron. 19, Supp 1, 213-217.Link
(18). Feuer, H., Leston, G., Miller, R., Nielsen, A. T. (1963). “Michael-type Reactions with α,α,ι,ι-Tetranitroalkanes.” The Journal of Organic Chemistry. 28, (2), 339–344.Link
(19). Archibald, T. G., Garver, L. C., Baum, K., Cohen, M. C. (1989). “Synthesis of polynitrocyclobutane derivatives.” The Journal of Organic Chemistry. 54, (12), 2869–2873.Link
(20). Griffin, G. W., Marchand, A. P. (1989). “Synthesis and chemistry of cubanes.” Chemical Reviews. 89, (5), 997–1010.Link
(21). Eaton, P.E., et al. (1997). "Synthesis and Chemistry of 1,3,5,7-Tetranitrocubane Including Measurement of Its Acidity, Formation of o-Nitro Anions, and the First Preparations of Pentanitrocubane and Hexanitrocubane." Journal of the American Chemical Society. 119, (41), 9591-9602.Link
(22). Agrawal, J.P. (1998). "Recent Trends in High-Energy Materials." Progress in Energy and Combustion Science. 24, (1), 1-30.Link